test: add toself fill + opacity mock

Author: etpinard

test/image/baselines/scatter_fill_self_opacity.png

@@ -0,0 +1,21 @@
+{
+  "data": [
+    {
+      "x": [
+        6,
+        7,
+        8
+      ],
+      "y": [
+        5,
+        6,
+        5
+      ],
+      "fill": "toself",
+      "opacity": 0.3
+    }
+  ],
+  "layout": {
+    "title": "toself fill with opacity"
+  }
+}